The organization of movements and associated postural adjustments depends critically upon the basal ganglia and tectum, particularly for orientation and attention related motor behavior which requires the coordination of eyes, head, neck and body. Recent anatomical and neurophysiological studies indicate that these two structures, usually viewed as independent systems, are closely linked by a loop system via the substantial nigra and intralaminar thalamus. It is the aim of the proposed experiments to determine anatomically and physiologically the relationship of the tectothalamic projection to its input from, and its output to the basal ganglia at the two critical synaptic interfaces: the deeper layers of the superior colliculus and the intralaminar thalamus. Modern anatomical techniques, such as retro- and anterograde transport of horseradish peroxidase and autoradiography will be applied to determine the nature of the synaptic linkage with nigrotectal and thalamo-neostriatal neurons. Other experiments will examine the topographic relationship of these projections, the extent of their overlap and the relationship to other intralaminar thalamic projections. The physiological experiments will analyze the sensory receptive fields and other functional properties of antidromically identified tectothalamic neurons with and without descending tectospinal axon collaterals. Their modulation by the inhibitory GABAergic negrotectal projection will be examined. Intracellular HRP labeling will be used to correlate neuronal morphology and function. These studies will provide information essential to the sensorimotor system linking basal ganglia, tectum and thalamus and the role of this system in normal orientation and attention related motor behavior and its disturbance in a wide range of movement disorders.